The invention is in the field of adjusting plate retainers, and is particularly useful for retaining the adjusting plates on the U.S. Army boresight collimater SU-93/TAS. This collimater, as currently made, requires partial disassembly for adjusting its various modes of operation. Since the collimater is normally purged with dry nitrogen after assembly, repurging becomes necessary. The collimater is thus not field adjustable, but must be sent to a maintenance depot for adjustment. Naturally, this takes the collimater out of service while it is being adjusted. The instant invention allows easy field adjustment without loss of purging.